Biaxial film

ABSTRACT

The invention relates to an optically biaxial film with cholesteric structure, methods and materials for its preparation, its use as retardation or compensation film in optical devices like liquid crystal displays, and to compensators and liquid crystal displays comprising such a biaxial retardation film.

FIELD OF INVENTION

The invention relates to an optically biaxial film with cholestericstructure, methods and materials for its preparation, its use in opticaldevices like compensators and liquid crystal displays, and to acompensator or liquid crystal display comprising such a biaxial film.

BACKGROUND AND PRIOR ART

Optical compensators are used in prior art to improve the opticalproperties of liquid crystal displays (LCD), such as the contrast ratioand the grey scale representation at large viewing angles. For examplein uncompensated displays of the TN or STN type at large viewing anglesoften a change of the grey levels and even grey scale inversion, as wellas a loss of contrast and undesired changes of the colour gamut areobserved.

An overview of the LCD technology and the principles and methods ofoptical compensation of LCDs is given in U.S. Pat. No. 5,619,352, theentire disclosure of which is incorporated into this application by wayof reference. As described in U.S. Pat. No. 5,619,352, to improve thecontrast of a display at wide viewing angles a negatively birefringentC-plate compensator can be used, however, such a compensator does notimprove the greyscale representation of the display. On the other hand,to suppress or even eliminate grey scale inversion and improve the greyscale stability U.S. Pat. No. 5,619,352 suggests to use a birefringentO-plate compensator. An O-plate compensator as described in U.S. Pat.No. 5,619,352 includes an O-plate, and may additionally include one ormore A-plates and/or negative C-plates.

The terms ‘O-plate’, ‘A-plate’ and ‘C-plate’ as used in U.S. Pat. No.5,619,352 and throughout this invention have the following meanings. An‘O-plate’ is an optical retarder utilizing a layer of a positivelybirefringent (e.g. liquid crystal) material with its principal opticalaxis oriented at an oblique angle with respect to the plane of thelayer. An ‘A-plate’ is an optical retarder utilizing a layer ofuniaxially birefringent material with its extraordinary axis orientedparallel to the plane of the layer, and its ordinary axis (also called‘a-axis’) oriented perpendicular to the plane of the layer, i.e.parallel to the direction of normally incident light. A ‘C-plate’ is anoptical retarder utilizing a layer of a uniaxially birefringent materialwith its extraordinary axis (also called ‘c-axis’) perpendicular to theplane of the layer, i.e. parallel to the direction of normally incidentlight.

Negative birefringent C-plate retarders in prior art have been preparedfor example from uniaxially compressed films of isotropic polymers, byvapour deposition of inorganic thin films, as described for example inU.S. Pat. No. 5,196,953, or from negatively birefringent liquid crystalmaterials. However, stretched or compressed polymer films often showonly moderate birefringence and require high film thickness, vapourdeposition requires complicated manufacturing procedures, and negativelybirefringent liquid crystal materials are often less easily availableand more expensive than positively birefringent materials.

To overcome these disadvantages, it has recently been suggested, forexample in WO 01/20393 and WO 01/20394, to use a cholesteric liquidcrystal film with short pitch, typically with its Bragg reflection bandin the UV region of the electromagnetic spectrum. Such a film exhibitsnegative birefringent C-type retardation for wavelengths greater thanits reflection maximum. The refractive index ellipsoid of this type offilm approximates to that of a vertically aligned liquid crystal withnegative birefringence. Such a retardation film can be used for exampleto cancel off-axis retardation in the homeotropically driven dark stateof a TN-LCD, and thus significantly improve the viewing angle of the LCdisplay.

WO 01/20393 discloses a compensator that is a combination of a planarA-plate, an O-plate and a negative C-plate, wherein the negative C-platecomprises a short-pitch cholesteric LC film. When used for example in aTN-LCD, this combination provides excellent contrast at horizontalviewing angles and reduces unwanted changes of the colour gamut.However, its performance at vertical viewing angles is limited.Furthermore, the use of multiple retardation films is expensive andraises manufacturing and durability problems.

One aim of the present invention is to provide an optical compensatorwhich has improved performance for compensation of LCDs, is easy tomanufacture, in particularly for mass production, and does not have thedrawbacks of prior art compensators as described above. Other aims ofthe present invention are immediately evident to the person skilled inthe art from the following detailed description.

The inventors have found that the above described problems can besolved, and an optical compensator with superior performance can beobtained, by combining multiple films in a single layer and by using abiaxial C-plate retarder. It was found that a biaxial negative C-plateretarder in its optical properties approximates to a combination of aplanar A-plate and a negative C-plate, but shows better opticalperformance than such a combination. The in-plane anisotropy of thebiaxial negative C-plate retarder (Δn_(xy)) approximates to the A-plateand the out-of-plane anisotropy (Δn_(xz) and Δn_(yz)) to the negativeC-plate. Simulations have shown that the optical performance of thebiaxial negative C-plate retarder is surprisingly superior to that ofthe A-plate and negative C-plate stacked sequentially, and showsexceptionally good viewing-angle performance for liquid crystaldisplays. Furthermore, the use of a single biaxial film instead of twostacked films reduces costs and manufacturing problems.

DEFINITION OF TERMS

In connection with optical polarisation, compensation and retardationlayers, films or plates as described in the present application, thefollowing definitions of terms as used throughout this application aregiven.

The term ‘cholesteric structure’ or ‘helically twisted structure’relates to a film comprising one or more layers of liquid crystalmaterial wherein the mesogens are oriented with their main molecularaxis in a preferred direction within molecular sublayers, with thispreferred orientation direction in different sublayers being twistedaround a helix axis that is substantially perpendicular to the filmplane, i.e. substantially parallel to the film normal. This definitionalso includes orientations where the helix axis is tilted at an angle ofup to 2° relative to the film normal.

The term ‘tilted structure’ or ‘tilted orientation’ means that theoptical axis of the film is tilted at an angle θ between 0 and 90degrees relative to the film plane.

The term ‘splayed structure’ or ‘splayed orientation’ means a tiltedorientation as defined above, wherein the tilt angle additionally variesmonotonuously in the range from 0 to 90°, preferably from a minimum to amaximum value, in a direction perpendicular to the film plane.

The term ‘planar structure’ or ‘planar orientation’ means that theoptical axis of the film is substantially parallel to the film plane.This definition also includes films wherein the optical axis is slightlytilted relative to the film plane, with an average tilt angle throughoutthe film of up to 1°, and which exhibit the same optical properties as afilm wherein the optical axis is exactly parallel, i.e. with zero tilt,to the film plane.

The average tilt angle θ_(ave) is defined as follows

$\theta_{ave} = \frac{\sum\limits_{d^{\prime} = 0}^{d}{\theta^{\prime}\left( d^{\prime} \right)}}{d}$wherein θ′(d′) is the local tilt angle at the thickness d′ within thefilm, and d is the total thickness of the film.

The tilt angle of a splayed film hereinafter is given as the averagetilt angle θ_(ave), unless stated otherwise.

The term ‘homeotropic structure’ or ‘homeotropic orientation’ means thatthe optical axis of the film is substantially perpendicular to the filmplane, i.e. substantially parallel to the film normal. This definitionalso includes films wherein the optical axis is slightly tilted at anangle of up to 2° relative to the film normal, and which exhibit thesame optical properties as a film wherein the optical axis is exactlyparallel, i.e. with no tilt, to the film normal.

For sake of simplicity, an optical film with a tilted, splayed, planar,twisted or homeotropic orientation or structure is hereinafter alsoshortly referred to as ‘tilted film’, ‘splayed film’, ‘planar film’,‘twisted film’ and ‘homeotropic film’, respectively.

Tilted and splayed films will also be referred to as ‘O plate’. A planarfilm will also be referred to as ‘A plate’ or ‘planar A plate’.

In tilted, planar and homeotropic optical films comprising uniaxiallypositive birefringent liquid crystal material with uniform orientation,the optical axis of the film as referred to throughout this invention isgiven by the orientation direction of the main molecular axes of themesogens of the liquid crystal material.

In a splayed film comprising uniaxially positive birefringent liquidcrystal material with uniform orientation, the optical axis of the filmas referred to throughout this invention is given by the projection ofthe orientation direction of the main molecular axes of the mesogensonto the surface of the film.

“E-mode” refers to a twisted nematic liquid crystal display (TN-LCD)where the input polarisation lies substantially along the director ofthe liquid crystal molecules when entering the display cell, i.e. alongthe extraordinary (E) refractive index. “O-mode” refers to a TN-LCDwhere the input polarisation lies substantially perpendicular to thedirector when entering the cell, i.e. along the ordinary (O) refractiveindex.

The term ‘film’ as used in this application includes self-supporting,i.e. free-standing, films that show more or less pronounced mechanicalstability and flexibility, as well as coatings or layers on a supportingsubstrate or between two substrates.

The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal ormesogenic compound’ should denote materials or compounds comprising oneor more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e.groups with the ability to induce liquid crystal phase behaviour. Thecompounds or materials comprising mesogenic groups do not necessarilyhave to exhibit a liquid crystal phase themselves. It is also possiblethat they show liquid crystal phase behaviour only in mixtures withother compounds, or when the mesogenic compounds or materials, or themixtures thereof, are polymerized.

For the sake of simplicity, the term ‘liquid crystal material’ is usedhereinafter for both liquid crystal materials and mesogenic materials,and the term ‘mesogen’ is used for the mesogenic groups of the material.

Polymerisable compounds with one polymerisable group are also referredto as ‘monoreactive’ compounds, compounds with two polymerisable groupsas ‘direactive’ compounds, and compounds with more than twopolymerisable groups as ‘multireactive’ compounds. Compounds without apolymerisable group are also referred to as ‘non-reactive’ compounds.

SUMMARY OF THE INVENTION

One object of the present invention is a biaxial film having acholesteric structure and a deformed helix with an elliptical refractiveindex ellipsoid, characterized in that it reflects light of a wavelengthof less than 380 nm.

Another object of the invention is a method of preparing a biaxial filmas described above and below.

Another object of the invention is the use of a biaxial film asdescribed above and below as retardation or compensation film in opticaldevices like for example liquid crystal displays.

Another object of the invention is a compensator comprising a biaxialfilm as described above and below.

Another object of the invention is a liquid crystal display comprising acompensator or biaxial film as described above and below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method of preparation of a biaxial film accordingto the present invention by photopolymerising a cholesteric materialusing polarised UV light.

FIG. 2 illustrates the production of a sinusoidal cholesteric helix (A)by photopolymerisation with unpolarised UV light, and of a distortedhelix (B) by photopolymerisation with polarised UV light.

FIG. 3 depicts the refractive index ellipsoids of a cholesteric materialwith non-distorted (A) and distorted (B) cholesteric helix.

FIG. 4 shows the retardation versus viewing angle of a cholesteric filmprepared according to example 1 by photopolymerisation with unpolarised(A) and polarised light (B).

FIG. 5 shows the retardation versus viewing angle of a cholesteric filmprepared according to example 2 by photopolymerisation with unpolarised(A) and polarised light (B).

FIG. 6 shows the retardation versus viewing angle of a cholesteric filmprepared according to example 3 by photopolymerisation with unpolarised(A) and polarised light (B).

FIG. 7 schematically depicts a compensated TN-LCD according to prior art(A, B) and according to the present invention (C).

FIGS. 8A, 8B and 8C show the isocontrast plots of compensated TN-LCDsaccording to example 4A, 4B and 4C, respectively.

FIG. 9 schematically depicts a compensated MVA-LCD according to priorart (A) and according to the present invention (B).

FIGS. 10A and 10B show the isocontrast plots of compensated MVA-LCDsaccording to example 5A, 5B and 5C, respectively.

FIG. 11 schematically depicts a compensated OCB-LCD according to priorart (A) and according to the present invention (B).

FIGS. 12A and 12B show the isocontrast plots of compensated OCB-LCDsaccording to example 6A, 6B and 6C, respectively.

DETAILED DESCRIPTION OF THE INVENTION

When using a compensator comprising a biaxial film according to thepresent invention in an LCD, the contrast at large viewing angles andthe grey level representation of the display are considerably improved,and grey scale inversion is suppressed. In case of coloured displays,the colour stability is considerably improved and changes of the colourgamut are suppressed. Furthermore, a compensator according to thepresent invention is particularly suitable for mass production.

Especially preferred is a biaxial retardation film which has opticallybiaxial negative C symmetry with n_(x)≠n_(y)≠n_(z) and n_(x),n_(y)>n_(z), wherein n_(x) and n_(y) are the principal refractiveindices in orthogonal directions in the film plane and n_(z) is theprincipal refractive index perpendicular to the film plane.

Further preferred is a biaxial retardation film which is substantiallytransparent for light with a wavelength of 380 nm or higher, preferablyfor visible light from 380 to 780 nm.

The thickness of the biaxial film is preferably from 0.5 to 5 μm, verypreferably from 1 to 3 μm.

The helical pitch is preferably chosen below 225 nm to achieve areflection wavelength of smaller than 360 nm which is below visiblewavelengths. The retardation of the biaxial film is preferably chosenaccording to the desired applications as exemplarily shown below and inthe examples.

Preferably the biaxial film comprises a crosslinked cholesteric polymer.

The biaxial film according to the present invention can be manufacturedfor example by inducing helix deformation in a cholesteric liquidcrystal (CLC) polymer film with short pitch (high twist). This can beachieved for example by photopolymerisation of a polymerisablecholesteric liquid crystal material that is coated onto a substrate andaligned into planar orientation, wherein the polymerisable materialcomprises a dichroic or liquid crystal photoinitiator andphotopolymerisation is initiated by irradiation with linear polarisedlight, e.g. linear polarised UV light. As a result, the CLC helix isdistorted during photopolymerisation. This method is described for thepreparation of a long-pitch CLC polymer film by D. J. Broer et al., Adv.Mater. 1999, 11(7), 573-77. However, Broer et al. do not disclosecholesteric films having a reflection wavelength in the UV region.

Thus, another object of the invention is a method of preparing a biaxialretardation film as described above and below, by providing a layer of achiral polymerisable liquid crystal material on a substrate,photopolymerising the polymerisable material that is homogeneouslyoriented in its liquid crystal phase by exposure to linear polarisedlight, and optionally removing the polymerised material from thesubstrate, wherein the chiral polymerisable liquid crystal materialcomprises at least one dichroic photoinitiator, at least one achiralpolymerisable, and at least one chiral polymerisable ornon-polymerisable compound.

Another object of the invention is a biaxial film with cholestericstructure obtainable by a method as described above and below.

The method of preparing a biaxial film is exemplarily described belowand schematically illustrated in FIGS. 1 and 2.

The CLC mixture preferably contains a highly reactive nematic componentand a low-reactive chiral component or vice versa. The LCphoto-initiator locally aligns with its UV-absorbing axis parallel tothe liquid crystal director. When illuminated with polarised UV light,polymerisation-initiating free radicals are predominantly produced wherethe local director lies parallel to the direction of polarisation (E),as depicted in FIG. 1.

Inhomogeneous free-radical production results in local polymerisation,predominantly of the highly reactive component. This results inconcentration gradients between the high and low reactive componentswithin a half turn of the helix, as shown in FIG. 2B. The highlyreactive components become concentrated where the director lies parallelto the E-field (maximum concentration of free radicals) and the lessreactive components where the director is perpendicular to the E-field.Localised variation of the chiral component results in distortion of thesinusoidal helix.

The previously reported, distorted helices have a long pitch in theorder of the wavelength of light in the material. The wavelength oflight in a cholesteric material is reduced by a factor of the refractiveindex compared to that outside the material. When the pitch of the helixequals the wavelength of light (inside the material) the Braggreflection occurs according to the equation p≈nxλ, wherein p is thecholesteric pitch, n the mean refractive index and λ the reflectionwavelength. The distorted helices produce Bragg reflection bands in thevisible spectrum in which linearly polarised light is transmitted,instead of circularly polarised light as usually observed in cholestericmaterials, due to helix distortion.

In the biaxial film according to the present invention the pitch isreduced values well below the visible wavelengths, so that only theaverage directional refractive indices are experienced. As a consequencethe Bragg reflection bands occur in the UV, so the film is transparentto visible wavelengths of light and behaves purely as retarders forthese wavelengths. Helix distortion in this case results in anelliptical, discotic refractive index ellipsoid (FIG. 3B) compared to acircular, discotic ellipsoid for a non-distorted helix (FIG. 3A). Incontrast, the films with longer pitch as reported by Broer et al., Adv.Mater. 1999, 11(7), 573-77 behave as polarised reflectors or colourfilters for visible wavelengths.

The short-pitch, sinusoidal (i.e. undistorted) helix in a cholestericfilm of prior art produces a negative effective birefringence(Δn_(z-xy)) as shown by the discotic refractive index ellipsoid in FIG.3A. The in-plane refractive indices are equal (n_(x)=n_(y)) and largerthan the out-of-plane index (n_(z)) This produces an optically uniaxial,negative C-type structure. In contrast, in the short-pitch cholestericfilms according to the present invention, helix distortion generatesadditional in-plane anisotropy (Δ_(n) _(x-y)) in the negative C-typestructure, resulting in a refractive index ellipsoid as shown in FIG. 3Bwith biaxial, negative C-type symmetry with n_(x)≠n_(y)≠n_(z) with n_(x)and n_(y) greater than n_(z).

In this way a cholesteric film with optical biaxial negative C-typesymmetry can be produced which can act as retarders for linear polarisedlight of wavelengths in the visible spectrum.

The biaxial film according to the present invention can be used alone orin combination with other retardation films as compensator for viewingangle compensation in LCDs.

Preferably the biaxial film is used in combination with an additionalretarder selected from the group of A-plate, C-plate and O-plateretarders or films having planar, homeotropic, tilted or splayedstructure. Especially preferably the biaxial film is used in combinationwith at least one O-plate retarder having tilted or splayed structure,very particularly preferably with splayed structure.

Another object of the invention is a compensator comprising at least onebiaxial retardation film as described above and below, and optionallyfurther comprising at least one O-plate retarder with splayed or tiltedstructure.

Suitable examples of O-plate retarders that can be used in a compensatoraccording to the present invention and their manufacture are describedin WO 01/20393, the entire disclosure of which is incorporated into thisapplication by reference.

The individual optical films like polarisers and retarders can belaminated together, or connected by means of adhesive layers, like forexample TAC or DAC (tri- or diacetylcellulose) films.

Another object of the present invention is a liquid crystal displaycomprising at least one biaxial film or compensator as described aboveand below.

Especially preferably the liquid crystal display device comprises thefollowing elements

-   -   a liquid crystal cell formed by two transparent substrates        having surfaces which oppose each other, an electrode layer        provided on the inside of at least one of said two transparent        substrates and optionally superposed with an alignment layer,        and a liquid crystal medium which is present between the two        transparent substrates,    -   a polariser arranged outside said transparent substrates, or a        pair of polarisers sandwiching said substrates, and    -   at least one biaxial film or compensator according to the        present invention, being situated between the liquid crystal        cell and at least one of said polarisers,        it being possible for the above elements to be separated,        stacked, mounted on top of each other or connected by means of        adhesive layers in any combination of these means of assembly.

The biaxial film and compensator according to the present invention canbe used for compensation of conventional displays, in particular thoseof the TN (twisted nematic), HTN (highly twisted nematic) or STN (supertwisted nematic) mode, in AMD-TN (active matrix driven TN) displays, indisplays of the IPS (in plane switching) mode, which are also known as‘super TFT’ displays, in displays of the DAP (deformation of alignedphases) or VA (vertically aligned) mode, like e.g. ECB (electricallycontrolled birefringence), CSH (colour super homeotropic), VAN or VAC(vertically aligned nematic or cholesteric) displays, MVA (multi-domainvertically aligned) displays, in displays of the bend mode or hybridtype displays, like e.g. OCB (optically compensated bend cell oroptically compensated birefringence), R-OCB (reflective OCB), HAN(hybrid aligned nematic) or pi-cell (π-cell) displays.

Especially preferred are TN, STN, VA, MVA, OCB and pi-cell displays.

In the following, compensated displays according to preferredembodiments of the present invention are described.

Computer simulations as described below are performed using the Berreman4×4 matrix method for stratified anisotropic media.

Twisted Nematic (TN) Mode

FIGS. 7A and 7B show a compensated TN display according to prior art,comprising an LC cell with a nematic liquid crystal mixture in twistednematic orientation in the off-state, a compensator comprising a planarA-plate, a (uniaxial) negative C-plate and a splayed O-plate on eachside of the cell, and two polarisers with their polarisation axescrossed at right angles sandwiching the cell and the compensators.

FIG. 7C exemplarily shows a compensated TN display according to a firstpreferred embodiment of the present invention, wherein, compared toFIGS. 7A and 7B, the compensator comprises a single biaxial negative Cfilm according to the present invention instead of separate A-plate andnegative C-plate retarders.

Computer simulations have shown that, in certain configurations, acompensator as shown in FIG. 7C significantly improves the opticalperformance of a TN display. The compensator configurations aredependent on the wave-guiding mode (O-mode or E-mode) and the relativeposition of the splayed and biaxial films. Modelling has also shown thatthe optical performance achieved with a compensator according to FIG.7C, comprising a single biaxial film plus a splayed film, can besignificantly better than that achieved with a compensator according toFIG. 7A or 7B, comprising separate A- and negative C-plates stackedsequentially with a splayed film.

In a compensation stack as shown for example in FIG. 7C, the ratio ofthe directional refractive indices of the inventive biaxial film is moreimportant than their magnitude. For example, in case of a biaxial filmwith n_(x)=1.65, n_(y)=1.55 and n_(z)=1.50, excellent contrast isachieved with a film thickness of 1200 nm.

However, it is also possible for example to reduce the in andout-of-plane anisotropy (Δn_(yz) and Δn_(xy)) by a factor, and tomultiply the film thickness by the same factor, to obtain a film withsubstantially the same optical performance. This method is applicable tothe biaxial films according to the present invention.

Multi-domain Vertically Aligned (MVA) Mode

Computer simulation has shown that a display of the MVA mode can becompensated to achieve 10:1 contrast ratio up to an angle of 80° in allviewing directions using a negative C-plate and an A-plate. This type ofcompensation also improves the colour performance, reducing the off-axiscolour washout.

FIG. 9A shows a compensated MVA display, comprising an LC cell with anematic liquid crystal mixture in homeotropic orientation in theoff-state, a compensator comprising a planar A-plate plus a (uniaxial)negative C-plate on one side of the LC cell, and two polarisers withtheir polarisation axes crossed at right angles sandwiching the cell andthe compensator.

FIG. 9B exemplarily shows a compensated MVA display according to asecond preferred embodiment of the present invention, comprising ahomeotropic LC cell and a biaxial negative C film according to theinvention on one side of the LC cell, sandwiched between two crossedpolarisers.

As previously described, the combination of a negative C-plate and anA-plate (planar film) can be approximated as a biaxial negative C film.Application of a single biaxial negative C film to a display of the MVAmode as shown in FIG. 9B surprisingly results in improved contrastcompared to the films applied separately as shown in FIG. 9A.

OCB or pi-Cell Mode

FIG. 11A shows a compensated OCB mode display, comprising an LC cellwith a nematic liquid crystal mixture with standard OCB configuration(homogeneous edge alignment and bent structure) in the off-state, acompensator comprising a planar A-plate plus a (uniaxial) negativeC-plate on each side of the LC cell, and two polarisers with theirpolarisation axes crossed at right angles sandwiching the cell and thecompensator.

FIG. 11B exemplarily shows a compensated OCB display according to athird preferred embodiment of the present invention, comprising an LCcell with bent structure, a biaxial negative C film according to theinvention on each side of the LC cell, sandwiched between two crossedpolarisers.

Computer simulations have shown that a single biaxial negative C film asshown in FIG. 11B can be used to replace a separate A-plate and negativeC-plate as shown in FIG. 11A to yield comparable optical performancewhile reducing the number of different films in the stack.

In the above described preferred embodiments, the A-plate is preferablya film of polymerised liquid crystal material with planar structure. Thenegative C-plate is preferably a film of polymerised liquid crystalmaterial with short-pitch cholesteric structure and reflection in the UVrange. The O-plate is preferably a film of polymerised liquid crystalmaterial with splayed structure. However, it is also possible to useother A-plate, C-plate and O-plate retarders known from prior art.Suitable films are disclosed for example in U.S. Pat. No. 5,619,352 orWO 01/20393.

The biaxial films according to the present invention can be preparedfrom polymerisable chiral liquid crystal materials that are developed toallow the reflection wavelength of the mixture to be below that of thelight that is normally used for polymerisation (typically at about 365nm) and to enable helix distortion. This is achieved for example byadding chiral components with high twist and/or in high amounts to pushthe Bragg reflection band into the UV, and for example to add a dichroicphotoinitiator to enable helix distortion. In addition, the mixtures andmaterials according to the present invention allow to make the filmproduction process suitable for manufacture on a plastic substrate, witha cure time of less than 5 minutes, which is especially suitable formass production.

The polymerisable material is preferably a cholesteric liquid crystal(CLC) material. Preferably it comprises one or more achiralpolymerisable mesogenic compounds and at least one chiral compound. Thechiral compounds can be selected from non-polymerisable chiralcompounds, like e.g. chiral dopants as used in liquid crystal mixturesor devices, polymerisable chiral non-mesogenic or polymerisable chiralmesogenic compounds. Especially preferred are chiral dopants that have ahigh helical twisting power, as they give short-pitch CLC mixtures evenif used in low amounts.

Especially preferred is a chiral polymerisable LC mixture comprising

-   a) at least one polymerisable mesogenic compound having at least one    polymerisable group,-   b) at least one chiral compound which may also be polymerisable    and/or mesogenic, and which may be one of the compounds of    component a) or an additional compound,-   c) at least one dichroic photoinitiator,-   d) optionally one or more non-mesogenic compound having one, two or    more polymerisable groups,-   e) optionally one or more non-dichroic photoinitiators,-   f) optionally one or more dyes showing an absorption maximum at a    wavelength used to initiate photopolymerisation,-   g) optionally one or more chain transfer agents, and-   h) optionally one or more surface-active compounds.

The chiral polymerisable LC materials as described above and below areanother object of the invention.

Preferably the achiral and chiral compounds have different number ofreactive groups.

In a preferred embodiment of the present invention the polymerisablemesogenic material comprises at least one di- or multireactive chiralpolymerisable mesogenic compound and at least one mono-, di- ormultireactive achiral polymerisable mesogenic compound and.

In another preferred embodiment of the present invention thepolymerisable material comprises at least one monoreactive chiralpolymerisable mesogenic compound and at least one mono-, di- ormultireactive achiral polymerisable mesogenic compound.

In another preferred embodiment the polymerisable material comprises atleast one non-reactive chiral compound and at least one mono-, di- ormultireactive polymerisable mesogenic compound.

If di- or multireactive compounds are present in the polymerisablematerial, a three-dimensional polymer network is formed. An opticalretardation film made of such a network is self-supporting and shows ahigh mechanical and thermal stability and a low temperature dependenceof its physical and optical properties.

By varying the concentration of the di- and multireactive compounds thecrosslink density of the polymer film and thereby its physical andchemical properties such as the glass transition temperature, which isalso important for the temperature dependence of the optical propertiesof the optical retardation film, the thermal and mechanical stability orthe solvent resistance can be tuned easily.

A preferred polymerisable LC mixture comprises

-   -   10-80% of one or more direactive achiral mesogenic compounds,    -   5-80% of one or more monoreactive achiral mesogenic compounds,    -   5-80% of one or more mono- or direactive chiral mesogenic        compounds, and/or 1-20% of one or more non-reactive chiral        compounds which may also be mesogenic,    -   0 to 10% of one or more chain transfer agents,    -   0 to 3% of one or more non-reactive, monoreactive, di- or        multireactive surfactants,    -   0.1 to 8% of one or more dichroic photoinitiators, preferably        0.5 to 5% of dichroic, very preferably liquid crystal,        photoinitiators,    -   0 to 6%, preferably 0.1 to 5% of one or more non-dichroic        photoinitiators.

The achiral and chiral polymerisable mesogenic mono-, di- ormultireactive compounds used for the instant invention can be preparedby methods which are known per se and which are described, for example,in standard works of organic chemistry such as, for example,Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.Typical examples are described for example in WO 93/22397; EP 0 261 712;DE 19504224; DE 4408171 and DE 4405316. The compounds disclosed in thesedocuments, however, are to be regarded merely as examples that do notlimit the scope of this invention.

Examples representing especially useful monoreactive chiral and achiralpolymerisable mesogenic compounds are shown in the following list ofcompounds, which should, however, be taken only as illustrative and isin no way intended to restrict, but instead to explain the presentinvention:

Examples of useful direactive chiral and achiral polymerisable mesogeniccompounds are shown in the following list of compounds, which should,however, be taken only as illustrative and is in no way intended torestrict, but instead to explain the present invention

In the above formulae, P is a polymerisable group, preferably an acryl,methacryl, vinyl, vinyloxy, propenyl ether, epoxy or stytryl group, xand y are each independently 1 to 12 , A is 1,4-phenylene that isoptionally mono- di or trisubstituted by L¹ or 1,4-cyclohexylene, v is 0or 1, Z⁰ is —COO—, —OCO—, —CH₂CH₂— or a single bond, Y is a polar group,Ter is a terpenoid radical like e.g. menthyl, Chol is a cholesterylgroup, R⁰ is an non-polar alkyl or alkoxy group, and L¹ and L² are eachindependently H, F, Cl, CN or an optionally halogenated alkyl, alkoxy,alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 Catoms.

The term ‘polar group’ in this connection means a group selected from F,Cl, CN, NO₂, OH, OCH₃, OCN, SCN, an optionally fluorinated carbonyl orcarboxyl group with up to 4 C atoms or a mono- oligo- or polyfluorinatedalkyl or alkoxy group with 1 to 4 C atoms. The term ‘non-polar group’means an alkyl group with 1 or more, preferably 1 to 12 C atoms or analkoxy group with 2 or more, preferably 2 to 12 C atoms.

The polymerisable material may also comprise one or morenon-polymerisable chiral dopants, which can also be mesogenic or liquidcrystalline. Especially preferred are compounds comprising a chiralsorbitol group with attached mesogenic groups, in particular compoundsas disclosed in WO 98/00428 with a high twisting power. Further suitablechiral compounds are e.g. the commercially available S 1011, R 811 or CB15 (from Merck KGaA, Darmstadt, Germany).

Very preferred are chiral compounds selected from the following formulae

including the (R,S), (S,R), (R,R) and (S,S) enantiomers not shown,wherein E and F have each independently one of the meanings of A givenabove, v is 0 or 1, Z⁰ is —COO—, —OCO—, —CH₂CH₂— or a single bond, and Ris alkyl, alkoxy, carbonyl or carbonyloxy with 1 to 12 C atoms.

The compounds of formula lit are described in WO 98/00428, the compoundsof formula IV are described in GB 2,328,207, the entire disclosure ofwhich is incorporated into this application by reference.

Further preferred chiral dopants are chiral binapthyl derivatives asdisclosed in EP 01111954.2, chiral binaphthol acetal derivatives asdescribed in EP 00122844.4, EP 00123385.7 and EP 01104842.8, chiralTADDOL derivatives as disclosed in EP 00115249.5, and chiral dopantswith at least one fluorinated bridging group and a terminal or centralchiral group as disclosed in EP 00115250.3 and EP 00115251.1.

For preparation of a cholesteric film, the polymerisable LC material ispreferably coated onto substrate, aligned into a uniform orientation andpolymerised to permanently fix the cholesteric structure. As a substratefor example a glass or quarz sheet or a plastic film or sheet can beused. It is also possible to put a second substrate on top of the coatedmixture prior to and/or during and/or after polymerisation. Thesubstrates can be removed after polymerisation or not. When using twosubstrates in case of curing by actinic radiation, at least onesubstrate has to be transmissive for the actinic radiation used for thepolymerisation. Isotropic or birefringent substrates can be used. Incase the substrate is not removed from the polymerized film afterpolymerisation, preferably isotropic substrates are used.

Preferably at least one substrate is a plastic substrate such as forexample a film of polyester such as polyethyleneterephthalate (PET) orpolyethylenenaphthalate (PEN), of polyvinylalcohol (PVA), polycarbonate(PC) or triacetylcellulose (TAC), especially preferably a PET film or aTAC film. As a birefringent substrate for example an uniaxiallystretched plastic film can be used. For example PET films arecommercially available from DuPont Teijin Films under the trade nameMelinex®.

The polymerisable material can also be dissolved in a solvent,preferably in an organic solvent. The solution is then coated onto thesubstrate, for example by spin-coating or other known techniques, andthe solvent is evaporated off before polymerization. In most cases it issuitable to heat the mixture in order to facilitate the evaporation ofthe solvent.

Polymerisation of the LC material is preferably achieved by exposing itto actinic radiation. Actinic radiation means irradiation with light,like UV light, IR light or visible light, irradiation with X-rays orgamma rays or irradiation with high energy particles, such as ions orelectrons. Preferably polymerisation is carried out by photoirradiation,in particular with UV light, very preferably with linear polarised UVlight. As a source for actinic radiation for example a single UV lamp ora set of UV lamps can be used. When using a high lamp power the curingtime can be reduced. Another possible source for photoradiation is alaser, like e.g. a UV laser, an IR laser or a visible laser.

Polymerisation is carried out in the presence of an initiator absorbingat the wavelength of the actinic radiation. For example, whenpolymerising by means of UV light, a photoinitiator can be used thatdecomposes under UV irradiation to produce free radicals or ions thatstart the polymerisation reaction. UV photoinitiators are preferred, inparticular radicalic UV photoinitiators.

For achieving helix distortion in the cholesteric film the polymerisableCLC mixture should preferably contain a dichroic photoinitiator, likefor example a liquid crystal photoinitiator. As LC photoinitiator forexample the following compound can be used:

In addition to the dichroic photoinitiators the polymerisable mixturemay also comprise one or more conventional photoinitators. As standardphotoinitiator for radical polymerisation for example the commerciallyavailable Irgacure® 651, Irgacure® 184, Darocure® 1173 or Darocure® 4205(all from Ciba Geigy AG) can be used, whereas in case of cationicphotopolymerisation the commercially available UVI 6974 (Union Carbide)can be used.

The curing time is dependent, inter alia, on the reactivity of thepolymerisable material, the thickness of the coated layer, the type ofpolymerisation initiator and the power of the UV lamp. The curing timeaccording to the invention is preferably not longer than 10 minutes,particularly preferably not longer than 5 minutes and very particularlypreferably shorter than 2 minutes. For mass production short curingtimes of 3 minutes or less, very preferably of 1 minute or less, inparticular of 30 seconds or less, are preferred.

The polymerisable LC material can additionally comprise one or moreother suitable components such as, for example, catalysts, sensitizers,stabilizers, chain-transfer agents, inhibitors, co-reacting monomers,surface-active compounds, lubricating agents, wetting agents, dispersingagents, hydrophobing agents, adhesive agents, flow improvers, defoamingagents, deaerators, diluents, reactive diluents, auxiliaries,colourants, dyes or pigments.

The mixture may also comprise one or more dyes having an absorptionmaximum adjusted to the wavelength of the radiation used forpolymerisation, in particular UV dyes like e.g. 4,4′-azoxy anisole orthe commercially available Tinuvin (from Ciba AG, Basel, Switzerland).

In another preferred embodiment the mixture of polymerisable materialcomprises up to 70%, preferably 1 to 50% of a monoreactive non-mesogeniccompound with one polymerisable functional group. Typical examples arealkylacrylates or alkylmethacrylates.

It is also possible, in order to increase crosslinking of the polymers,to add up to 20% of a non-mesogenic compound with two or morepolymerisable functional groups to the polymerisable LC materialalternatively or in addition to the di- or multireactive polymerisablemesogenic compounds to increase crosslinking of the polymer. Typicalexamples for direactive non-mesogenic monomers are alkyldiacrylates oralkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typicalexamples for multireactive non-mesogenic monomers aretrimethylpropanetrimethacrylate or pentaerythritoltetraacrylate.

It is also possible to add one or more chain transfer agents to thepolymerisable material in order to modify the physical properties of theinventive polymer film. Especially preferred are thiol compounds, suchas monofunctional thiol compounds like e.g. dodecane thiol ormultifunctional thiol compounds like e.g. trimethylpropanetri(3-mercaptopropionate), very preferably mesogenic or liquidcrystalline thiol compounds. When adding a chain transfer agent, thelength of the free polymer chains and/or the length of the polymerchains between two crosslinks in the inventive polymer film can becontrolled. When the amount of the chain transfer agent is increased,the polymer chain length in the obtained polymer film is decreasing.

For preparing the cholesteric film, it is necessary to achieve planaralignment of the chiral polymerisable material, i.e. with the helicalaxis being oriented substantially perpendicular to the plane of thefilm. Planar alignment can be achieved for example by shearing thematerial, e.g. by means of a doctor blade. It is also possible to applyan alignment layer, for example a layer of rubbed polyimide or sputteredSiO_(x), on top of at least one of the substrates. Planar alignment canalso be achieved by rubbing the substrate without applying an additionalalignment layer, e.g. by means of a rubbing cloth or a rubbing roller.Planar alignment with a low tilt angle can also be achieved by addingone or more surfactants to the polymerizable mesogenic material.Suitable surfactants are described for example in J. Cognard, Mol.Cryst. Liq. Cryst. 78, Supplement 1, 1-77 (1981). Particularly preferredare non-ionic surfactants, e.g. non-ionic fluorocarbon surfactants, likethe commercially available Fluorad® (from 3M), or Zonyl FSN® (fromDuPont).

In some cases it is of advantage to apply a second substrate to aidalignment and exclude oxygen that may inhibit the polymerisation.Alternatively the curing can be carried out under an atmosphere of inertgas. However, curing in air is also possible using suitablephotoinitiators and high UV lamp power. When using a cationicphotoinitiator oxygen exclusion most often is not needed, but watershould be excluded. In a preferred embodiment of the invention thepolymerisation of the polymerisable material is carried out under anatmosphere of inert gas, preferably under a nitrogen atmosphere.

The examples below serve to illustrate the invention without limitingit. In the foregoing and the following, all temperatures are given indegrees Celsius, and all percentages are by weight, unless statedotherwise.

EXAMPLES Manufacturing of Biaxial CLC Films Example 1

The following polymerisable mixture was prepared

Compound (1) (monoreactive chiral) 63.0% Compound (2) (direactiveachiral) 20.0% Compound (3) (monoreactive achiral) 7.8% Compound (4)(non-reactive chiral) 5.0% Compound (5) (chain transfer agent) 2.0%Compound (6) (dichroic photoinitiator) 2.0% FC171 ® (surfactant) 0.2%

FC171® is a non-polymerisable fluorocarbon surfactant commerciallyavailable from 3M (St. Paul, Minn., USA). The preparation of the chiraldopant (4) is described in EP 01111954.2.

The mixture was dissolved in 7:3 toluene/cyclohexanone to give a 50% w/wsolution. A PVA coated TAC (triacetyl cellulose) substrate was preparedby rubbing. The solution was coated onto the substrate using awire-wound bar to give a wet film of approximately 10 μm. The solventwas allowed to evaporate and a second PVA coated TAC substrate placed ontop. The resultant coating was polymerised at 80° C. by exposing to 0.8mWcm⁻² of unpolarised UV (365 nm) irradiation to give film 1A. A secondcoating was prepared in the same way and polymerised by exposing tolinearly polarised UV (365 nm) irradiation to give film 1B. Theretardation (nm) versus viewing angle (degrees) of films 1A and 1B isshown in FIGS. 4A and 4B, respectively. The retardation of film 1A issubstantially independent of the viewing-angle. The in-plane anisotropyof film 1B is represented by the on-axis retardation (˜10 nm) in FIG.4B. The out-of plane, negative C retardation of film 1B is evident inFIG. 4B from the off-axis reduced retardation in all directions.

Example 2

The following polymerisable mixture was prepared

Compound (7) (direactive achiral) 51.5% Compound (8) (monoreactiveachiral) 8.0% Compound (9) (monoreactive achiral) 21.0% Compound (2)12.0% Compound (4) 6.0% Compound (6) 1.0% FC171 ® 0.2%

The mixture was dissolved in 7:3 toluene/cyclohexanone to give a 50% w/wsolution. A PVA coated TAC (triacetyl cellulose) substrate was preparedby rubbing. The solution was coated onto the substrate using awire-wound bar to give a wet film of approximately 6 μm. The solvent wasallowed to evaporate and a second PVA coated TAC substrate place on top.The resultant coating was polymerised at 80° C. by exposing to 0.8mWcm⁻² of unpolarised UV (365 nm) irradiation to give film 2A. A secondcoating was prepared in the same way and polymerised by exposing tolinearly polarised UV (365 nm) irradiation to give film 2B. Theretardation (nm) versus viewing angle (degrees) of films 2A and 2B isshown in FIGS. 5A and 6B, respectively. The retardation of film 2A issubstantially independent of the viewing-angle. The in-plane anisotropyof film 2B is represented by the on-axis retardation (˜10 nm) in FIG.5B. The out-of plane, negative C retardation of film 2B is evident inFIG. 5B from the off-axis reduced retardation in all directions.

Example 3

The following polymerisable mixture was prepared

Compound (7) 50.5% Compound (8) 8.0% Compound (9) 19.0% Compound (2)10.0% Paliocolor LC756 ® (reactive chiral) 6.0% Compound (6) 2.0%FC171 ® 0.5%

Paliocolor LC756® is a direactive polymerisable chiral compoundcommercially available from BASF AG (Ludwigshafen, Germany).

The mixture was dissolved in 7:3 toluene/cyclohexanone to give a 50% w/wsolution. A TAC (triacetyl cellulose) substrate was prepared by rubbing.The solution was coated onto the substrate using a wire-wound bar togive a wet film of approximately 5 μm. The solvent was allowed toevaporate and a second TAC substrate placed on top The resultant filmwas polymerised at 25° C. by exposing to 40 mWcm⁻² of unpolarised UV(365 nm) irradiation to give film 3A. A second coating was prepared inthe same way and polymerised by exposing to linearly polarised UV (365nm) irradiation to give film 3B. The retardation (nm) versus viewingangle (degrees) of films 3A and 3B is shown in FIGS. 6A and 6B,respectively. The retardation of film 3A is substantially independent ofthe viewing-angle. The in-plane anisotropy of film 3B is represented bythe on-axis retardation (˜10 nm) in FIG. 6B. The out-of plane, negativeC retardation of film 3B is evident in FIG. 6B from the off-axis reducedretardation in all directions.

Use Examples Compensation of LCDs with Biaxial CLC Films

The following abbreviations are used:

-   θ_(max): maximum tilt angle-   θ_(min): minimum tilt angle-   out: =outwards, surface of the film facing the polarisers-   in: =inwards, surface of the film facing the LC cell-   d: film thickness-   OA: orientation direction of the stretch axis in case of a    polariser, the optical axis in case of a retardation film, the LC    molecules at the surface of an LC cell, the direction of n_(x) in    case of a biaxial film

Example 4A Comparison Example TN-LCD

A compensated TN-LCD with a configuration as shown in FIG. 7A and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA=45°-   O-Plate 1: splayed structure, θ_(max) (out) 88°, θ_(min) (in) 2°,    linear tilt gradient, OA=225°, d=1332 nm, n_(o)=1.50, n_(e)=1.62-   A-Plate 1: OA=135°, d=1222 nm, n_(o)=1.50, n_(e)=1.62-   −C-Plate 1: d=896 nm, n_(o)=1.56, n_(e)=1.50-   LC Cell: d=4750 nm, OA=45°(1), 135°(2), O-mode (=orientation at each    surface (1,2) parallel to stretch axis of respective nearest    polariser(1,2)), standard TN director distributions-   −C-Plate 2: d=896 nm, n_(o)=1.56, n_(e)=1.50-   A-Plate 2: OA=225°, d=1222 nm, n_(o)=1.50, n_(e)=1.62-   O-Plate 2: splayed structure, θ_(max) (out) 88°, θ_(min) (in) 2°,    linear tilt gradient, OA=135°, d=1332 nm, n_(o)=1.50, n_(e)=1.62-   Polariser 2: stretched type, OA=315°    and has an isocontrast plot as shown in FIG. 8A.

Example 4B Comparison Example TN-LCD

A compensated TN-LCD with a configuration as shown in FIG. 7B and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA=45°-   O-Plate 1: splayed structure, θ_(max) (out) 88°, θ_(min) (in) 2°,    linear tilt gradient, OA=225°, d=1093 nm, n_(o)=1.50, n_(e)=1.62-   −C-Plate 1: d=1000 nm, n_(o)=1.56, n_(e)=1.50-   A-Plate 1: OA=135°, d=954 nm, n_(o)=1.50, n_(e)=1.62-   LC Cell: d=4750 nm, OA=45°(1), 135°(2), O-mode, standard TN director    distributions-   A-Plate 2: OA=225°, d=954 nm, n_(o)=1.50, n_(e)=1.62-   −C-Plate 2: d=1000 nm, n_(o)=1.56, n_(e)=1.50-   O-Plate 2: splayed structure, θ_(max) (out) 88°, θ_(min) (in) 2°,    linear tilt gradient, OA=135°, d=1093 nm, n_(o)=1.50, n_(e)=1.62-   Polariser 2: stretched type, OA=315°    and has an isocontrast plot as shown in FIG. 8B.

Example 4C Use Example TN-LCD

A compensated TN-LCD with a configuration as shown in FIG. 7C and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA 45°-   O-Plate 1: splayed structure, θ_(max) (out) 88°, θ_(min) (in) 2°,    linear tilt gradient, OA=225°, d=1200 nm, n_(o)=1.50, n_(e)=1.62-   Biaxial film1: OA=225°, d=1100 nm, n_(x)=1.643, n_(y)=1.541,    n_(z)=1.495-   LC Cell: d=4750 nm, OA=45°(1), 135°(2), O-mode, standard TN director    distributions-   Biaxial film2: OA=135°, d=1100 nm, n_(x)=1.643, n_(y)=1.541,    n_(z)=1.495-   O-Plate 2: splayed structure, θ_(max) (out) 88°, θ_(min) (in) 2°,    linear tilt gradient, OA=135°, d=1200 nm, n_(o)=1.50, n_(e)=1.62-   Polariser 2: stretched type, OA=315°    and has an isocontrast plot as shown in FIG. 8C.

Example 5A Comparison Example MVA-LCD

A compensated MVA-LCD with a configuration as shown in FIG. 9A and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA=90°-   A-Plate: OA=90°, d=725 nm, n_(o)=1.50, n_(e)=1.62-   −C-Plate: d=2500 nm, n_(o)=1.56, n_(e)=1.50-   LC Cell: d=3000 nm, four vertically aligned domains (45°, 135°,    225°, 315°), standard MVA director distributions-   Polariser 2: stretched type, OA=0°    and has an isocontrast plot as shown in FIG. 10A.

Example 5B Use Example MVA-LCD

A compensated MVA-LCD with a configuration as shown in FIG. 9B and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA=90 °-   Biaxial film1: OA=90°, d=2277 nm, n_(x)=1.583, n_(y)=1.610,    n_(z)=1.495-   LC Cell: d=3000 nm, four vertically aligned domains (45°, 135°,    225°, 315°), standard MVA director distributions-   Polariser 2: stretched type, OA=0°    and has an isocontrast plot as shown in FIG. 10B.

Example 6A Comparison Example OCB-LCD

A compensated OCB-LCD with a configuration as shown in FIG. 11A and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA=45°-   A-Plate 1: OA=90°, d=265 nm, n_(o)=1.50, n_(e)=1.62-   −C-Plate 1: d=4655 nm, n_(o)=1.56, n_(e)=1.50-   LC Cell: d=4000 nm, OA=0° (1), 180° (2), standard OCB director    distributions-   −C-Plate 2: d=4655 nm, n_(o)=1.56, n_(e)=1.50-   A-Plate 2: OA=90°, d=265 nm, n_(o)=1.50, n_(x)=1.62-   Polariser 2: stretched type, OA=315°    and has an isocontrast plot as shown in FIG. 12A.

Example 6B Use Example OCB-LCD

A compensated OCB-LCD with a configuration as shown in FIG. 11B and abacklight on top of the stack has the following parameters

-   Polariser 1: stretched type, OA=45°-   Biaxial film1: OA=90°, d=960 nm, n_(x)=1.865, n_(y)=1.615,    n_(z)=1.446-   LC Cell: d=4000 nm, OA=0° (1), 180° (2), standard OCB director    distributions-   Biaxial film2: OA=90°, d=960 nm, n_(x)=1.865, n_(y)=1.615,    n_(z)=1.446-   Polariser 2: stretched type, OA=315°    and has an isocontrast plot as shown in FIG. 12B.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A biaxial film having a cholesteric structure and a deformed helixwith an elliptical refractive index ellipsoid, which reflects light of awavelength of less than 380 nm.
 2. A biaxial film according to claim 1,having n_(x) and n_(y) as the principal refractive indices in orthogonaldirections in the film plane and n_(z) as the principal refractive indexperpendicular to the film plane, wherein n_(x)≠n_(y)≠n_(z) and n_(x),n_(y)>n_(z).
 3. A biaxial film according to claim 1, which issubstantially transparent for light with a wavelength of 380 nm orhigher.
 4. A biaxial film according to claim 1, which comprises acrosslinked cholesteric polymer.
 5. A biaxial film according to claim 1,obtainable by providing a layer of a chiral polymerizable liquid crystalmaterial on a substrate, photopolymerizing the polymerizable materialthat is homogeneously oriented in its liquid crystal phase by exposureto linear polarized light, and optionally removing the polymerizedmaterial from the substrate, wherein the chiral polymerizable liquidcrystal material comprises at least one dichroic photoinitiator and atleast one achiral polymerizable and at least one chiral polymerizable ornon-polymerizable compound.
 6. A biaxial film according to claim 5,wherein the chiral polymerizable liquid crystal material comprises a) atleast one polymerizable mesogenic compound having at least onepolymerizable group, b) at least one chiral compound which may also bepolymerizable and/or mesogenic, and which may be one of the compounds ofcomponent a) or an additional compound, c) at least one dichroicphotoinitiator, d) optionally one or more non-mesogenic compounds havingone, two or more polymerizable groups, e) optionally one or morenon-dichroic photoinitiators, f) optionally one or more dyes showing anabsorption maximum at a wavelength used to initiate photopolymerization,g) optionally one or more chain transfer agents, and h) optionally oneor more surface-active compounds.
 7. A biaxial film according to claim5, wherein the chiral polymerizable liquid crystal material comprises atleast one monoreactive chiral polymerizable mesogenic compound and atleast one mono-, di- or multireactive achiral polymerizable mesogeniccompound.
 8. A biaxial film according to claim 5, wherein the chiralpolymerizable liquid crystal material comprises at least one di- ormultireactive chiral polymerizable mesogenic compound and at least onemono-, di- or multireactive achiral polymerizable mesogenic compound. 9.A biaxial film according to claim 5, wherein the chiral polymerizableliquid crystal material comprises at least one non-reactive chiralcompound and at least one mono-, di- or multireactive achiralpolymerizable mesogenic compound.
 10. A method for preparing a biaxialfilm as described in claim 5, comprising providing a layer of a chiralpolymerizable liquid crystal material on a substrate, photopolymerizingthe polymerizable material that is homogeneously oriented in its liquidcrystal phase by exposure to linear polarized light, and optionallyremoving the polymerized material from the substrate, wherein the chiralpolymerizable liquid crystal material comprises at least one dichroicphoto initiator and at least one achiral polymerizable and at least onechiral polymerizable or non-polymerizable compound.
 11. A method forproviding a retardation or compensation film in an optical device orliquid crystal display, comprising providing for said device or liquidcrystal display a biaxial film according to claim
 1. 12. A compensatorcomprising at least one biaxial film according to claim
 1. 13. Acompensator according to claim 12, further comprising at least oneretardation film with splayed or tilted structure.
 14. A liquid crystaldisplay comprising at least one biaxial film according to claim 1, or acompensator comprising said at least one biaxial film and at least oneretardation film with spayed or tilted structure.
 15. A liquid crystaldisplay comprising the following elements a liquid crystal cell formedby two transparent substrates having surfaces which oppose each other,an electrode layer provided on the inside of at least one of said twotransparent substrates and optionally superposed with an alignmentlayer, and a liquid crystal medium which is present between the twotransparent substrates, a polarizer arranged outside said transparentsubstrates, or a pair of polarizers sandwiching said substrates, and atleast one biaxial film according to claim 1 or a compensator comprisingsaid at least one biaxial film, being situated between the liquidcrystal cell and at least one of said polarizers, it being possible forthe above elements to be, each independently, separated, stacked,mounted on top of each other or connected by an adhesive layer.
 16. Aliquid crystal display according to claim 14, which is a display of theTN (twisted nematic), OCB (optically compensated bend), pi-cell, VA(vertically aligned) or MVA (multi-domain vertically aligned) mode. 17.A biaxial film according to claim 1, which is substantially transparentfor light with a wavelength of visible light of 380 to 780 nm.